1. Field of the Invention
My invention relates to the field of message enciphering and deciphering, and more particularly to key generating by autokey techniques.
2. Description of the Prior Art
Autokey encipherment or encryption refers generally to a substitution cipher in which the key, following the application of an initial key, is determined in whole or in part by preceeding elements of the key or cipher. The most common autokey systems include those known by the terms key autokey and cipher text autokey. In a key autokey encryptor, each key bit generated is a function of one or more prior key bits. Key in a cipher text autokey encryptor is a function of a prior sum of key and plain text bits, that sum comprising the enciphered message.
An important advantage of an autokey system is its ability to self-synchronize; i.e., the receiving key generator is not required to be preset to a previously determined value prior to receiving and decoding cipher messages. Cipher text autokey systems continuously self-synchronize throughout the transmission, while key autokey systems may be initially synchronized by temporarily operating in a CTAK mode until synchronization is achieved. Inherent in prior autokey systems is the feature of limited error extension. That is, a transmission error affects the key generated by the receiver for only a limited number of steps while the bit in error passes through the key generator register, after which the error no longer has any effect. The reverse of limited error extension would be infinite error extension, i.e., an incorrectly received data bit would be inserted into the receiver key generator with the result that all succeeding key bits could be affected.
An advantage of infinite error extension is its capability to insure to a high degree of probability that a message has been received without error. For example, a message to be enciphered might have a plurality of zeros or ones appended to it prior to encipherment and transmission. The receiver would decipher the message, utilizing the received signal to develop the decipherment key. The recovered message would contain the string of appended zeros or ones only if each bit in the enciphered message had been received without error, thereby insuring accuracy to a very high degree of probability. In general, it is well known that m check bits can give at most 2−m protection, i.e., the likelihood that an incorrectly received message will test out as correct is ½m. An encipherment system employing a 2n state key generator can give at most 2−n protection. A system utilizing both of the above will provide protection against an error going undetected with a probability equal to the lesser of 2−m or 2−n.
Heretofore, infinite error extension has not been available in any form having acceptable cryptographic characteristics. My invention overcomes this deficiency in the art of enciphered message communications by providing an enciphering/deciphering system having infinite error extension plus a high degree of cryptographic security.